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Police in Kenya's capital, Nairobi, intercepted a suspicious car at a traffic light Wednesday and were taking the occupants in for questioning when the car exploded, killing four people, including two police officers, officials said.

The car exploded just outside a police station's main gate in central Nairobi, turning the car into an unrecognizable heap of twisted metal.

David Kimaiyo, the inspector general of police, said two officers were killed, along with two other people.

Terror warnings have been a constant in Kenya in recent months, particularly after September's attack on Westgate Mall killed at least 67 people. Most of the small-scale attacks and explosions are blamed on al-Shabab, the militant group in Somalia that has vowed revenge attacks in Kenya because Kenyan troops moved into southern Somalia in 2011.

Patrick Kimiti was close to the car when it exploded. "I was just tossed up immediately and when I landed, I lay on the ground. There was so much dust. People were screaming. Cars were skidding and making U-turns," he said.

Kenyan authorities this month have been carrying out an extensive sweep of Nairobi's ethnic Somali neighbourhoods, making thousands of arrests and deporting more than 100 people to Somalia.

Somali Kenyans say they know that further police clamp-downs will follow attacks such as Wednesday's car bomb blast.